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Hard coatings used to protect engineering components from external loads and harsh environments should ideally be strong and tough. Here we study the fracture toughness, K_IC, of Ti_1−xAl_xN upon annealing by employing micro-fracture experiments on freestanding films. We found that K_IC increases by about 11% when annealing the samples at 900 °C, because the decomposition of the supersaturated matrix leads to the formation of nanometer-sized domains, precipitation of hexagonal-structured B4 AlN (with their significantly larger specific volume), formation of stacking faults, and nano-twins. In contrast, for TiN, where no decomposition processes and formation of nanometer-sized domains can be initiated by an annealing treatment, the fracture toughness K_IC remains roughly constant when annealed above the film deposition temperature. As the increase in K_IC found for Ti_1−xAl_xN upon annealing is within …